I am stuck on the following problems. I don't know how to 'plug in,' or substitute, the correct way--please show me how.

1) The yield V is in millions of cubic feet and x is the number of years. What is the time needed to yield 2.1 million cubic feet?

V(x)=6.7(e^-48.1/x)

2) With buying power B and t years, when will B(t) be half of what it is today?

B(t)=(0.92)^t

For this one my teacher said that since the original buying power was 1 I had to change it to .5. She said I should put the .5 in front of the (0.92) but I still don't know what to do from there.

Also for this one logarithm, I think I've done it right, can you verify?

1) logx^2 = -10

x^2 = 10^-10

x = 100000